CAN I LOOP THROUGH TEXTBOX ID’S TO CHECK VALUES IN ASP.NET

I'm working on a project and I'm a beginner and I'm having a
little trouble with this. I'm trying to check if a textbox is empty and if it
is to change the value to N/A so that I can input n/a into a database instead
of it not working.

Here is the code that I thought would work but didn't
because the .Text property isn't near the ID anymore:

for(int i = 1; i<=17; i++)

{

if(!("tb" + i).Text)

"tb" + i.Text = "n/a";

}

I wasn't sure if the true/false would work but I never got
to find out because it doesn't compile to begin with. I have 17 textboxes on my
design page all with ID 'tb + i' e.g tb1, tb2